In the petroleum refining art for upgrading of heavy petroleum fractions, it is frequently the practice to direct such fractions to a delayed coking unit to produce coke and lighter hydrocarbon products. Typical of such heavy petroleum fractions, also referred to as heavy petroleum residua, is the bottoms fraction from a vacuum distillation tower. Vacuum distillation towers generally are used to further fractionate the bottoms fraction from a crude oil atmospheric tower. Other fractions which can be furthered processed in a delayed coker unit include the bottoms residuum from the main fractionation of a catalytic cracker, and other residua having an initial boiling temperature of about 430.degree. C. or higher. These heavy petroleum residua have generally a high content of sulfur and heavy metals, which render them unsuitable for fluid catalytic cracking because of their tendency to foul and deactivate the catalysts. The coke made from these heavy petroleum residua using a conventional delayed coking unit has a high content of sulfur, heavy metals and in the instance when the bottoms from a catalytic cracker is used the coke may also contain catalyst leftover material such as silica and alumina.
The delayed coking process is a well known refinery process. In a typical delayed coking process, a high boiling residuum is heated to very high temperatures to extract the last usable hydrocarbons in an acceptable boiling range such as light naphtha, diesel or light fuel oil, leaving as a final residue, a solid coke containing from about 85% to about 96% carbon.
More specifically, in a delayed coking process, which is essentially a high severity thermal cracking, the heavy oil feedstock is heated rapidly in a fired heater or tubular furnace from which it flows directly to a large coking drum which is maintained under conditions at which coking occurs, generally with temperatures above about 450.degree. C. under a slight superatmospheric pressure. In the drum, the heated feed decomposes to form coke and volatile components which are removed from the top of the drum and passed to a fractionator. When the coke drum is full of solid coke, the feed is switched to another drum and the full drum is cooled and emptied of the coke product. Generally, at least two coking drums are used so that one drum is being charged while coke is being removed from the other.
When the coking drum is full of solid coke, the hydrocarbon vapors are purged from the drum with steam. The drum is then quenched with water to lower the temperature to about 93.degree. C. after which the water is drained. When the cooling step is complete, the drum is opened and the coke is removed by hydraulic mining or cutting with high velocity water jets. A high speed, high impact water jet cuts the coke from the drum. A hole is bored in the coke from water jet nozzles located on a boring tool. Nozzles oriented horizontally on the head of a cutting tool cut the coke from the drum.
There are basically three different types of solid coke products which are different in value, appearance and properties. They are needle coke, sponge coke and shot coke. Needle coke, also known as anode grade or premium coke, is the highest quality of the three varieties. Needle coke, upon further treatment, has high conductivity and is used in electric arc steel production. It is low in sulfur and metals and is typically produced from some of the higher quality coker charge stocks which include more aromatic feedstocks such as slurry and decant oils from catalytic crackers and thermal cracking tars as opposed to the asphaltenes and resins.
Sponge coke, a lower quality coke, sometimes called "regular coke," is most often formed in refineries. Low quality refinery coker feedstocks having significant amounts of asphaltenes, heteroatoms and metals produce this lower quality coke. If the sulfur and metals content is low enough, sponge coke can be used for the manufacture of electrodes for the aluminum industry. If the sulfur and metals content is too high, then the coke can only be used as a cheap fuel. The name "sponge coke" comes from its porous, sponge-like appearance.
Shot coke is the lowest quality coke because it has the highest sulfur and metals content, the lowest electrical conductivity, and is the most difficult to grind. The name shot coke comes from the shape which is similar to that of B--B sized balls. The shot coke has a tendency to agglomerate into larger masses, sometimes as much as a foot in diameter, which can cause refinery equipment and processing problems. Shot coke is made from the lowest quality high resin-asphaltene feeds and makes a good high sulfur fuel source. It can also be used in cement kilns and steel manufacture.
While conventional delayed coking processes can convert a wide variety of petroleum residues, the product quality depends upon the type of feedstock used. Generally, low quality feeds produce low quality coke and liquid and gaseous products, having a high content of sulfur, heavy metals, and other inorganic contaminants. Existing processes for making high purity coke utilize higher quality feedstocks having low sulfur and heavy metals content or include treating the feedstock to remove these contaminants prior to the coking step.
For instance, U.S. Pat. No. 5,695,631, issued to Eguchi, et al., describes a process for making needle coke by reducing the ash content of a heavy oil residuum to less than 0.01 wt % and subsequently coking the thus treated heavy oil. U.S. Pat. No. 4,178,229, issued to McConaghy, et al., describes a process for making needle coke from a heavy hydrocarbon material such as vacuum residue, by subjecting it to a hydrogen donor diluent cracking operation ("HDDC"), fractionating the effluent from the HDDC process, and using the pitch from the fractionator as feedstock to a premium coker unit.
Also, the required equipment is generally expensive, since conventional coking processes involve handling solids and heat transfer at very high temperatures. Often, as described in detail above, the coke is formed in bulk and must be recovered by use of expensive hydraulic cutting equipment.
U.S. Pat. Nos. 3,179,584, 3,803,023, 5,258,115 and 5,466,361 describe coking processes involving the use of alkali metal compounds. However, none of these processes is suitable for making high purity, anode grade coke, which is the object of the present invention.
U.S. Pat. No. 3,179,584, issued to Hammer, describes a coking process utilizing alkali metal compounds in order to increase the hydrogen content of a coker's gaseous products. U.S. Pat. No. 3,803,023, issued to Hammer, describes a process using an alkali metal containing coke produced in a coking zone which is subsequently steam treated in a separate gasification zone to produce a hydrogen-containing gas and the remaining coke is recycled to the coking zone as seed coke.
U.S. Pat. No. 5,258,115, issued to Heck, et al., describes a process for recycling caustic waste which consists of introducing caustic waste (spent caustic) to a delayed coking unit during coking of a conventional coker feedstock. Finally, U.S. Pat. No. 5,466,361, issued to Heck, et al., describes a process for disposing caustic waste, which consists of co-injecting the caustic waste with a coker feedstock, and the subsequent gasification of the resulting coke product.
Therefore, the problem of producing high purity coke, substantially free from sulfur and heavy metals directly from a heavy petroleum residuum having a high content of sulfur and metals, remains unsolved.